(1) Field of the Invention
The present invention generally relates to sonar systems and methods and, more specifically, to a matched-field based sonar system and method that supports real-time, passive, three-dimensional acoustic-source localization using a mobile horizontal array.
(2) Description of Prior Art
Sonar systems detect and locate underwater objects, including missiles, torpedoes, and submarines. Active sonar systems transmit and receive in a set direction, while passive systems listen to all angles at all times. Passive sonar systems are mainly used in military applications because they are silent, have a much greater range than active sonar, and allow identification of a target through target motion analysis (TMA). TMA determines the target's trajectory (i.e. target's range, course, and speed). Thus far, TMA has been performed using own-ship maneuvers and triangulation, e.g., marking the direction from which sound comes at different times and comparing the motion with that of the operator's ship. Changes in relative motion are analyzed using standard geometrical techniques and assumptions about limiting cases. This process is extremely time-consuming and requires manual interaction. Also, in order to calculate speed, the operator must know certain contact characteristics that are acquired over time.
Passive sonar is typically deployed in the form of towed arrays to enhance detection of sound sources. Towed sonar arrays are sonar systems that are designed to be towed by a submarine or a surface vessel in order to detect other submarines or objects. The arrays are typically long, hose-like structures measuring up to a thousand feet or longer that contain specially designed acoustic sensors (hydrophones) that receive acoustic waves. The arrays include electronics that convert the acoustical waves from analog to digital form and transmit that data to electronic processors on board the towing vessel.
One of the most important features of an array is that the array allows for beamforming which can be used for acoustic source localization. Since these are passive systems they must listen to all angles at all times and this requires a number of beams. At the same time, a narrow beamwidth is required for locating the source and rejecting ambient noise. These two objectives are achieved simultaneously by a passive beamforming processor. The input/output of each transducer is put through the beamforming processor, which applies time delays or phase shifts to each of the signals in such a way as to create a narrow beam in a particular direction. This increases the gain in the direction of wanted signals and decreases the gain in the direction of interference and noise. Depending on the speed of the towing platform, a towed array may display a level of cant (i.e. inclination to the horizontal plane). Sonar processors that disregard the effect of array cant will erroneously report contacts at an erroneous bearing.
A number of approaches to beamforming and other acoustic-source localization techniques exist. These include plane wave beamforming, range-focused beamforming, multipath-ranging technique, and matched field processing (MFP). Of these, plane wave beamforming is the most mature and is the easiest to implement. For a single line towed array, it provides only bearing information however, and accuracy diminishes as the acoustic source moves from broadside to end-fire. Range focused beamforming improves on this by additionally providing range estimates. However, these range estimates have very coarse resolution and like plane wave beamforming, this technique has performance degradation at end-fire. Both plane wave and range focused beamforming cannot resolve the port/starboard ambiguity issues when using a linear array and neither provide depth estimates.
Another method for estimating range is the multipath ranging technique, which is strictly a manual process. Given a bearing-time display, the operator must calculate range based on the various acoustic propagation paths associated with a particular contact. This process is tedious, prone to error, and requires the operator to know the acoustic environment. The process also only works in specific environments. All of the acoustic-source localization techniques described thus far also do not work well in shallow water.
Matched field processing is another localization technique. This technique is very computational and expensive, and to date this has prohibited use in real-time applications. Systems incorporating matched-field processing thus far have been restricted to the laboratory where very small data sets are processed over very restricted search regions (bearing, range, depth, and frequency). Generally, these systems also have utilized vertical stationary arrays, rather than horizontal mobile arrays.
A number of patent efforts have been directed to point source localization systems and methods and include the following.
U.S. Pat. No. 5,357,484 issued to Bates et al. on Oct. 18, 1994 discloses an acoustic source localization method which determines the range and depth to an acoustic source from a sampling site in a medium, with the aid of a linear array of pressure transducers; measurement, back propagation, and index processors; and an environmental model.
U.S. Pat. No. 5,481,505 issued to Donald et al. on Jan. 2, 1996 discloses an acoustic source localization method which utilizes matched field processing of measured data. This method detects, processes, and tracks sonar signals to provide bearing, range, and depth information that locate an object in a three-dimensional underwater space. An inverse beamformer utilizes signals from a towed horizontal array of hydrophones to estimate a bearing to a possible object. A matched field processor receives measured covariance matrix data based upon signals from the hydrophones and from a propagation model. A peak picker provides plane wave peaks in response to output beam levels from the matched processor. A tracker identifies peaks within the specified limit of frequency, bearing change over time, range and depth to specify an object as a target and to display its relative range and depth with respect to the array of hydrophones.
Although the above-described acoustic source localization methods as well as others have furthered technological development, none of the methods provide a low-cost system which incorporates a computationally efficient matched-field processing algorithm supporting real-time processing, a global bathymetry database allowing the system to operate in any oceanographic location, and other features including dynamic array shape compensation and high resolution range and bearing estimates.
Thus it would be greatly advantageous to provide a sonar system capable of matched field processing (MFP) that (1) supports real-time, passive, three-dimensional acoustic-source localization using a mobile horizontal array; (2) improves the performance of extant sonar systems by providing continuous localization estimates in bearing, range, and depth; (3) supports port/starboard ambiguity resolution; (4) improves acoustic sensitivity at forward and aft end-fire as well as in shallow water; (5) provides a real-time target motion analysis capability; (6) has minimal space requirements making it suitable for shipboard deployment on both surface ships and submarines; and (7) is of low cost.